Australasian Science: Australia's authority on science since 1938

Legless on Land

By Stephen Luntz

In another blow to proponents of Intelligent Design, clues have been found to the evolution of one of the world’s oddest fish, the Pacific leaping blenny (Alticus arnoldorum).

“This terrestrial fish spends all of its adult life living on the rocks in the splash zone, hopping around defending its territory, feeding and courting mates. They offer a unique opportunity to discover in a living animal how the transition from water to the land has taken place,” says Dr Terry Ord of the UNSW School of Biological, Earth and Environmental Sciences.

The fish appears uniquely unsuited to the ecological niche it occupies since it must move on land without legs and can never afford to dry out as it breathes through moisture on its gills. Its method of motion is to curve its tail around and press hard against the ground, causing it to spring into the air.

Ord says that leaping blennies have colonised rocky locations on islands across the Pacific, and even on Mauritius. “All environments are a complex mix of niches, and these are not necessarily saturated by one species,” he says. “Had there been some lizard or crustacean filling this niche perhaps the blenny would not have been able to grab it.”

The blenny is well-camouflaged, its green and brown markings blending into the rocks. To test the value of this attire Ord made carefully coloured plasticine blenny models and placed them on rocks and nearby white sand.

“After several days we collected the models and recorded how often birds, lizards and crabs had attacked them from the marks in the plasticine. We found the models on the sand were attacked far more frequently than those on the rocks,” Ord says.

Having unsurprisingly found that the blenny’s camouflage protects them, Ord investigated its origins. Related species of fish living nearby have similar colouring, probably because they lay eggs in holes in similar rocks.

“We know there are several problems the blennies would have faced in coming out of the water,” says Ord. Already having the right colouring to escape predators made the improbable shift easier. Not all environments have lots of predators, but if the first blennies moving onto land had to deal with many generalist predators as well as learning to breathe and tail-hop it is reasonable to think they might not have made it without camouflage.

The remaining question is what drove the blennies onto land. Ord ponders if the predators in the ocean were actually worse, if the algae that now forms their main diet was simply too tempting, or if some fish were stranded by waves and made the best of it.